Two trains, one of an engine and a car and
another - of an engine and two cars, meet at a segment of railroad.
There is a switch or side-track on that segment of railroad - just as
shown in the illustration. The switch is large enough only to hold one
engine or one car at a time.

The object is, using only this switch, to exchange the trains in order
they can continue their journeys and do that in the most expeditious
way. No other outside help, except the switch itself, is allowed.
Please, note that a car cannot be connected to the front of an engine.